The present invention relates generally to the field of wafer processing reactors or systems and methods used in the manufacture of semiconductors and integrated circuits. More specifically, the invention relates to an atmospheric pressure wafer processing reactor having an internal pressure control system and method.
Wafer processing reactor systems and methods are widely used in the manufacture of semiconductors and integrated circuits. One particular type of wafer processing system utilizes chemical vapor deposition (CVD) to deposit films or layers on the surface of a substrate as a step in the manufacture of semiconductors and integrated circuits. A variety of different CVD systems are used in the art. For example, films may be deposited using low pressure CVD (LPCVD) systems, atmospheric pressure CVD (APCVD) systems or different types of plasma enhanced CVD (PECVD) systems. In general principle, all such systems employ a deposition chamber where certain injected gaseous chemicals react and deposit a layer of material on the surface of the substrate. Many types of materials may be deposited, with dielectrics such as oxides and doped oxides being a typical example.
For proper operation of the system, and in particular to deposit a film of desired quality and repeatability, the flow of the gases within the reactor is important. Specifically, it is desirable to achieve a substantially uniform flow of gases in the area proximate the surface of the substrate so that certain concentrations of the gaseous chemicals or reactants are available at the surface of the substrate to deposit a proper film. Moreover, control of the flow of such gases promotes more efficient utilization of the gases for reaction.
Another important criterion when depositing films is the thickness uniformity of the film. It is desirable to achieve a film of substantially uniform thickness over the entire surface of the substrate. This aspect becomes even more important as the diameter of substrates continues to increase. The flow of the reactive gases within the chamber plays an important role in the resulting film thickness. Thus, it is desirable to control the flow rate of the gases and to promote substantially uniform flow of the reactive gases over the entire surface of the substrate.
A further important criterion in wafer processing systems is minimization of particles and contaminants formed in the reactor. Particles and contaminants are caused mainly by the accumulation of unreacted and by-product gaseous chemicals and the formation of deposits (often called powder build-up) on interior reactor surfaces. These deposits are a substantial source of particles that may contaminate the films deposited on the substrate. To remove the deposits the system must be taken offline and serviced. Contaminants and gaseous chemicals that accumulate in stagnant flow regions promote corrosion of the reactor and can severely reduce system longevity, as well as contribute to the contamination problem. The flow of inert and reactive gases plays an important role in either promoting or minimizing the accumulation of unreacted and by-product gaseous chemicals, and thus determines, in part, the extent of the powder build-up. It is therefore desirable to provide a system that promotes control of the inert and reactive gas flows to minimize accumulation and powder build-up.
It has been found that the control of the exhaust flow rate of the various gases may be used to address the aforementioned concerns. Problems may arise when the exhaust system of a reactor does not function properly. For example, if the exhaust flow rate is too high, reactive gases do not completely react and deposition on the surface of the substrate is hampered. Conversely, if the exhaust flow rate is too slow, the gas flows are undefined, leading to increased accumulation in the chamber that may cause deposits to form on the chamber walls. Accordingly, it is desirable to provide a system and method that controls, or xe2x80x9cmeters,xe2x80x9d the exhaust flow of gases by achieving and maintaining certain selected gas flow rate values within the system. Additionally, since powder build up does occur and may lead to changes in flow conditions over time, it is desirable to provide a system and method which employs control means which accurately control the gas flows and do not deteriorate over time.
One prior art approach that has addressed these issues is described in U.S. Pat. No. 6,143,080, the disclosure of which is hereby incorporated by reference. In general the ""080 patent provides a wafer processing system for delivering a processing gas and an inert gas to a chamber that includes a CVD processing region having a plurality of gas flow paths for conveying the gases to the chamber and exhausting them from the chamber. The active exhaust of the bypass plenums allows excess chamber gases to be extracted from the system without asymmetric gas flow conditions surrounding different chambers. Placement of the load and unload exhausts internal to the system and between inert gas curtains allows the exhaust flow control system to actively maintain a desired pressure differential (near zero) across the chambers in an open APCVD system exposed to changing external environmental conditions. The system exhaust flow control system merges load and unload gas paths and the bypass exhaust gas flow path into the chamber exhaust gas flow path.
In this exemplary prior art system and method, a flow control system is coupled to each of several exhaust gas flow paths. Each of the process gas exhaust flow paths are separately controlled to maintain a constant rate of flow within each of the gas flow paths independent of the accumulation of deposition byproducts. A self-cleaning orifice is utilized to facilitate a pressure differential measurement in the process exhaust line to measure flow. The wafer processing system is provided with load and unload regions surrounding the chamber(s), each having additional inert gas exhaust flow paths. While this prior art system and method of gas flow control has provided advances in the field, additional improvements are desirable. For example, in-the prior art system, the self-cleaning orifice is subject to glass and powder accumulation that distorts the assumed correlation between exhaust gas mass flow and pressure difference across the orifice over time. Thus, the total exhaust gas mass flow may change over time, causing an undesirable shift in the deposited oxide film thickness and variation in process results from one wafer to the next. For SiO2 film applications, the process exhaust typically drops as the orifice accumulates glass and powder over time and the deposited film thickness increases. Additionally, due to the requirement for self-cleaning of the orifice, mechanisms to rotate the toroid and the spring that wipes the orifice surface allow leakage in the exhaust line. Because the resultant seal is less than optimal, thorough leak-checking of the facility exhaust line is inhibited. Furthermore, leaks may shift the assumed correlation of exhaust gas mass flow and pressure difference across the orifice. Thus, after preventive maintenance disassembly and cleaning of the exhaust line components, the setpoint for the pressure difference across the orifice must be modified frequently to achieve the same process condition. Maintaining the same setpoints on the system is desirable for production operation without extra engineering support.
Finally, controlling the total exhaust mass flow to be constant in an open Atmospheric Pressure CVD system does not compensate for input gas flow changes, nor does it maintain a stable pressure balance in the system when the external conditions change. Variations in input gas flow through rotameters occur when facility gas supply pressures change, as can happen when gas flows in adjacent non-continuous processing systems are turned on or off. Also, when operators open or close the portal doors to access wafer cassettes for loading or unloading, the load end of the system is exposed to the clean room pressure which may be substantially above the chase side pressure surrounding the system. Wafer loading of the system also affects the internal pressure balance, particularly since very small pressure variations can perturb the gas flows and deposited film results inside an open APCVD system. Accordingly, it is desirable to provide further improvements in atmospheric pressure wafer processing systems.
It is an object of the present invention to provide an improved wafer processing system, and more particularly an improved atmospheric pressure chemical vapor deposition (APCVD) system. Another object of the present invention is to provide a system and method that minimizes accumulation of gases and the formation of unwanted deposits within the system. A further object of the present invention is to provide a system and method that promotes the deposition of substantially uniform films on the surface of substrates.
In one embodiment, these and other objects of the invention are achieved by an atmospheric pressure wafer processing system for delivering at least one gas, having an exhaust control feedback system that utilizes sensitive sensors to measure the differential pressure within the system relative to the chase ambient pressure and adjusts control units to maintain the selected preset pressures within the system. In particular, the sensors measure the pressures inside a muffle, and specifically the load, bypass center and unload sections of the muffle, relative to the chase ambient pressure. Controlling the muffle pressures directly within the atmospheric system yields a more stable pressure balance for processing wafers less subject to changes in the external environment and allows for compensation of varying input gas flows as occurs when the supply pressure to the system may vary.
In another embodiment of the present invention, a chemical vapor deposition processing system is provided for delivering one or more reactive gases and one or more inert gases to process a wafer or other substrate. The system comprises a muffle. a load region through which wafers are inserted into the muffle, an unload region through which wafers are removed from the muffle, and a process chamber exhaust flow path through which reactive gases and some inert gases removed from the muffle are exhausted. The muffle is maintained at approximately atmospheric pressure and contains at least one process chamber that houses at least one injector through which one or more reactive gases are injected and at least one shield or curtain through which one or more inert gases are injected into a deposition region and at least one exhaust vent through which reactive gases and inert gases are removed. At least a first pressure transducer is provided for measuring the pressure difference between the muffle in the process chamber region and the ambient atmospheric pressure and providing a feedback control signal in response to the pressure difference. A first control unit comprising a first throttle valve that is controllable in response to the feedback control signal meters the flow of gases that are exhausted from the process chamber exhaust flow path of the CVD system.
In another embodiment of the present invention, a method of delivering and exhausting a gas to a process chamber in an atmospheric pressure reactor is provided. In general, the method comprises the steps of establishing at least one gas flow path within the reactor. The gas is conveyed through a gas flow path and the differential pressure between the process chamber or one or more sections of the gas flow path surrounding the process chamber and the ambient atmosphere outside the system is measured. A flow control unit, responsive to the measured differential pressure or pressures, is selectively adjusted to control a flow rate of the gas to maintain the differential pressure substantially equal to a preset, constant value. Additionally, the gas may travel through a plurality of gas flow paths, and the flow rates of the gases are separately controlled to maintain selected differential pressures within each of the measured areas. Further, additional gas flow rates may be maintained at a substantially constant value within each of the gas flow paths to promote uniform delivery and exhaust of the gases, even if the gas temperature or geometry of the path changes over time.